加入on 的話就變成強(qiáng)調(diào)句式了it is . that ...屬于強(qiáng)調(diào)句式 去掉的話不會(huì)影響句子結(jié)構(gòu)和意義也就是 our class decide to plant trees on April 12th 仍然是完整句式,但是主語從句 不可以 如本句： 如果去掉的話 ...那應(yīng)該怎么寫看你怎樣表達(dá)了 如下：who decide to plant trees on April 12this our class. - 形式主語： It is our class who decide to plant trees.what our class decide to do on April 12 th is toplant trees. when our class decide to plant trees is April 12th.It is April 12th when our class decide to plant trees.主語不是特別長的情況下沒有必要使用形式主語。